1. The Field of the Art
The present invention relates to a control circuit which compensates focus error, tracking error or the like in an optical recording and reproducing apparatus.
2. Background of the Prior Art
The use of optical recording and reproducing apparatuses using semiconductor lasers has increased and an apparatus having both recording-reproducing functions as well as reproducing is now produced on a commercial scale. There are opto-magnetic methods, optical methods, or the like as recording-reproducing methods; a surface state of a disk is varied by raising laser light output in the recording process, and recorded information is read by a low laser light output in the reproducing process. In order to record and reproduce the information by the laser light, precise tracking of the laser light along a desired recording track are required. These respective requirements are realized by a servo system for focus control and tracking control, respectively.
Focus control generally will now be further described. As shown in FIG. 1, laser light beam is radiated from laser optical system 11 and is impinged onto a disc 13. Reflected light beam from the disc 13 reaches a PIN photodiode pair 15 through a beam splitter 12. Output signals of the PIN photodiode pair 15 are fed to operational amplifiers 16 and 17. A focus error which represents the focus error of the laser light on the disc 13 and a sum signal which represents the light magnitude of the laser light beam are obtained from the PIN photodiode pair 15 and the operational amplifiers 16 and 17. The focus error signal whose, reference level is 0 V generally, is used in a focus control system. Even though focus error signals have the same value V in the reproducing process and the recording process, they represent different focus error values depending on the state involved. That is to say, provided that the focus error is n .mu.m for the error signal v in the reproducing process, the focus error is n/4 .mu.m even if the error signal is the same v in the recording process, because the output value of the laser light beam of the laser optical system 11 is made four times as great in the recording process. Loop gain of the focus control loop is maintained constant by having a feedback system that obtains the amount of focus error precisely in the recording process and the reproducing process, by using a division circuit 18 to obtain a stable focus servo system.
The numeral 19 designates a phase compensating circuit, and it compensates the phase delay of a actuator 14 which drives the laser optical system 14 for focussing, whereby a loop gain point of the feedback loop that becomes a value of 1 is set to a stable frequency. Numeral 20 is a pull-in signal generator, which moves the laser optical system 11 in saw tooth wave manner by the actuator 14 and searches for an accurately focused position in order to pull-in the focus. A switching circuit 21 supplied to the drive circuit 22 a signal of the pull-in signal generator 20 until completion of pull-in of the focus, and after completion of pull-in of the focus, supplies a signal of the phase compensation circuit 19 for controlling operation of the focus control. The drive circuit 22 drives the actuator 14 in response to the signal from the switching circuit 21, and controls the laser optical system 11 through the actuator 14 to control focusing.
As mentioned above, the division circuit 18 is essential and important in the optical disk apparatus which records and reproduces datas on the optical disk 13 by using the feedback control loop system for focussing, and various division circuits have been proposed. An example of the conventional division circuit is shown in FIG. 2. The division circuit in FIG. 2 uses a multiplier 30 and an operational amplifier 31, and it is made by IC which is on sale. (Reference document: ICL8013 in a manual of Intersil, MC1495L, MC1594L in the manual of Motorola linear/interface devices).
Referring to FIG. 2, a sum signal of light magnitude is inputted through a terminal 33 to the multiplier 30. The focus error signal is inputted through an input terminal 34 and passes a resistor 32. The focus error signal and the output of the multiplier 30 are inputted to the operational amplifier 31. The output of the operational amplifier 31. The output of the operational amplifier 31 is fed back to the multiplier 30. The division output signal is outputted from the amplifier 31 through an output terminal 35.
Next, operation of the circuit as shown in FIG. 2 will be described as follows. The output of the multiplier 30 is formed as a current source and its output current (I.sub.O) is ##EQU1##
Since the output of the operational amplifier 31 is fed back to the multiplier 30 the multiplier 30 is controlled to equalize two inputs of the operational amplifier 31. One input of the operational amplifier 31 is grounded, and the both inputs become 0 V. Accordingly, provided that the resistance of the resistor 32 is R, EQU (Focus error signal)=I.sub.O.R (2).
The division output signal is obtained from the equations (1) and (2) as follows: ##EQU2##
This shows that the focus error signal (z input signal) is divided by sum signal of light magnitude (x input signal).
In the division circuit of this system, however, to remove the offsets of three inputs, the x input signal (sum signal of light magnitude), z input signal (focus error signal) and division output signal, which are fed back to the multiplier 30, three variable resistors for offset adjustment are required. In order to improve precision of division, trimming of the resistor formed in an integrated circuit is required, and a special process to provide this on an integrated circuit is necessary. Therefore, for the conventional division circuit, although merchandized as an independent integrated circuit, it is not suitable for mounting on an integrated circuit for automatic control.